How do you calculate velocity pressure from air velocity?

Velocity pressure (VP) in IWG = (V / 4005)^2, where V is air velocity in FPM at standard air density (0.075 lb/ft3). For 1,000 FPM: VP = (1000/4005)^2 = 0.0622 IWG. Conversely, velocity from VP: V = 4005 x sqrt(VP). This relationship comes from Bernoulli's equation and is the basis for Pitot tube airflow measurements. At non-standard air conditions (high altitude, high temperature), the air density correction factor must be applied.

How do I measure CFM with a Pitot tube?

A duct traverse measures velocity pressure at multiple points across the duct cross-section, then averages them to find mean velocity. ASHRAE recommends a log-linear traverse pattern with 16-25 measurement points for rectangular ducts. For each point: velocity = 4005 x sqrt(VP). Average all velocities, then: CFM = average velocity x duct area (ft2). The traverse method is more accurate than a single centerline reading because velocity is not uniform across the duct — it is highest at the center and lower near the walls.

🔋 Velocity Pressure

Duct Velocity Pressure Calculator

Convert between air velocity (FPM) and velocity pressure (IWG) instantly. Run a full Pitot tube duct traverse to calculate mean velocity and CFM. Used by HVAC technicians for TAB commissioning and system diagnostics. Pairs with the static pressure calculator for complete duct measurements.

⚙ Calculation Mode

📈 Inputs

Velocity Pressure *

IWG

Air Velocity *

FPM

Air Temperature

°F

Affects air density correction

Altitude (optional)

ft ASL

Calgary: 3,500 ft. Denver: 5,280 ft.

Duct Area (for CFM)

in²

Use duct area calculator

🔋 Velocity Pressure Results

Parameter	Value	Note

Export:

Velocity pressure: the foundation of duct airflow measurement

Every Pitot tube measurement, every duct traverse, and every digital manometer reading in HVAC fieldwork converts between velocity and velocity pressure using the same formula: VP = (V/4005)². Understanding this relationship is essential for TAB (Testing, Adjusting, and Balancing) technicians, energy auditors, and anyone diagnosing HVAC systems that aren't delivering design airflow.

Why you need a duct traverse, not a centerline reading

Air velocity is not uniform across a duct cross-section. At the duct centerline, velocity can be 20-30% higher than the average. A single centerline Pitot tube reading will overestimate actual CFM by the same margin. ASHRAE Standard 111 requires a multi-point traverse with readings taken at the centroid of equal-area sections. For rectangular ducts, a minimum 25-point grid is recommended. This calculator supports up to 25 traverse points and performs the proper square-root averaging method per ASHRAE. Use the air velocity calculator to check whether your measured velocity is within standard duct velocity limits.

Air density correction for Calgary and high-altitude cities

The standard 4005 constant assumes air at 70°F at sea level (density 0.075 lb/ft³). In Calgary at 3,500 ft, air density is about 0.068 lb/ft³ — 9% less. A Pitot tube reading that indicates 1,000 FPM at sea level indicates only 940 FPM in Calgary for the same velocity pressure reading, because the thinner air produces less velocity pressure at the same speed. This calculator applies the density correction automatically when you enter temperature and altitude.

Frequently Asked Questions

What is the difference between static pressure and velocity pressure?+

Static pressure (SP) is the pressure a fluid exerts perpendicular to the direction of flow — it pushes outward on duct walls. Velocity pressure (VP) is the pressure associated with the kinetic energy of moving air — it acts only in the direction of flow. Total pressure = SP + VP. A Pitot tube measures total pressure at its forward-facing port and static pressure at its side ports; the difference is velocity pressure. In duct design, static pressure drives the friction loss calculations. In airflow measurement, velocity pressure is converted to velocity for CFM calculations. The static pressure calculator covers the static pressure side of system design.

My manometer shows negative velocity pressure — what does that mean?+

A negative velocity pressure reading almost always means the Pitot tube is oriented backwards — the total pressure port is facing downstream instead of upstream. Reverse the tube so the tip faces into the airflow. Velocity pressure is always a positive value (it represents kinetic energy, which cannot be negative). If you are measuring static pressure and get a negative reading, that is normal and expected in return ducts and upstream of the fan — it indicates the duct is below atmospheric pressure, which is what draws return air in. Use the air balancing calculator to convert your traverse CFM measurements into a balancing report.